growth of colonies after ligation

hello guys
i have a very curious question.i did several set of ligation using the same set of plasmid and the insert and did transformation.i tried to screen colonies containing the ligated product in amp plate.after12 hrs of incubation i didnot get any colonies but i just kept these plates on my desk and after 1 day i saw colonies in 3 plates but no colonies in the rest of the 2 plates.
how come that i got colonies in 3 plates ? is the mode of action of amp bacteriostatic? i dont understand how can colonies develop like this from no where.do the cells grow after the amp degrades in the plate?